More than any other international conflict before or after, the Vietnam War was the one that photography ended. The images of military brutality brought, to a global audience, the terrifying reality of war and the consequences for its victims. Their influence, first on public opinion and then on foreign policy, was as severe as it was lasting.

Of the thousands of journalists responsible for capturing those images, possibly the most distinguished was a Welshman with a lazy eye. Philip Jones Griffiths used to be the night manager at Boots in Piccadilly. He spent two decades reporting from the front line because he found pharmacy impossibly dull. "Never underestimate the power of boredom," Griffiths says. "Bored men have done more for world peace than most."

Now 72 and one of the most respected photographers in the world, Griffiths is dying of cancer. "Excuse me," he regularly interjects during this interview, "that'll be the tumour in my lung. It's pressing into my trachea, and it hurts. I'm on my last legs, you know."

Born in 1936 in the tiny village of Rhuddlan, near Rhyl, in North Wales, Griffiths remembers the Second World War – "not so exciting when you're stuck in a shelter at the bottom of the garden" – and the excitement of news arriving from around the country.

"My father used to buy the Picture Post each week. I read it like the New Testament. Some of the images I saw there are still with me," he says. "They fuelled a kind of itching curiosity in me, something every good journalist is born with. I needed to know, to see, to feel. But they also gave me a scepticism: I realised that without actually being inside the photo, without seeing for myself, I'd never totally believe what I saw."

Rationing during the war led to his eye problem, which ironically came to his advantage. "I had what we used to call a lazy eye. It meant that if I wanted to check contrast on a photo, I could use my sharper left eye, and compare it with my softer right eye."

He asked to put on a night shift at Boots. "That way I could take photos during the day. Hard work, but it changed my life," he says. He started selling his work to The Guardian, the first national newspaper to give picture credits. Soon noticed by the picture desk at The Observer, he began working for them full-time at the start of 1962.

By the middle of that year he was being sent abroad, and soon had his first major scoop, uncovering the brutal treatment of Algerian nationalists by the French military. "I still regard that as my biggest scoop," he says. When, in 1965, trouble began to brew in Vietnam, he was an obvious choice to be sent there.

"I had just joined Magnum [the photo agency he went on to become president of], who gave me a fantastic press pass. I tried several times to get into the North of the country, but no journalists managed it. Instead I roamed the South, first visiting every province on my own, and then attaching myself to the military."

Griffiths resists the idea that he was "embedded" with the Americans. "That's a modern term. I used to travel with them but never felt I had to stick to their path. Half my time I spent living with poor families in Saigon – the people the Americans often killed."

But his access to the military gave him every foreign correspondent's crutch: mobility. "Road was impossible, so we had to fly, always to where the action was," he says. That it provided him with regular scoops was a bonus: "It was surreal: as domestic support for the war waned, the American GIs would be sitting there smoking pot, feeding me all these scoops because they hated what they were doing."

Long before the days of digital cameras and the internet, Griffiths developed his own photographs. "There were no zoom-lenses back then, so you had to take four cameras everywhere." Equipped with two Leicas and two Nikons, Griffiths would send occasional photos back by wire, though it was "expensive and unreliable". "There was nothing you could do about the time-delay; you had to rely on air-freight."

Always aware "the images in my viewfinder were revolutionary", he denies that he has political motives – "only human ones" – though, when asked why he never married, he says, "I'd never let bourgeois society dictate my behaviour."

He's as forthcoming about the moral power of his work. "Journalism is about obliterating distances, bringing far away things closer home and impressing it on people's senses. You excite your humanity every time you take a photo; lose your humanity and you stop being able to judge, to know, to see."

Griffiths' best work in Vietnam is captured in his book, Vietnam Inc, published first in 1971, and then again in 2001 with an introduction by Noam Chomsky. "If anybody in Washington had read that book, we wouldn't have had these wars in Iraq or Afghanistan," he declares.

"My tools as a photographer are different to other reporters. I don't have the time or space they have to go into context; my images speak alone, which can be a problem. But all journalists share two concerns: first, for truth; second, for the suffering of innocents. No man can see what I've seen and not be moved to tell others about it."